1. Field of the Invention
The present invention relates to steering control devices that output an operation signal to a steering device that can change the steering angle ratio, namely the ratio between the steered angle of steered wheels of a vehicle and the steering angle, or the rotation angle of a steering.
2. Description of the Related Art
Conventionally, in steering devices, there is an upper limit value permitted for the steered angle of the steered wheels. For example, in those steering devices having a rack and pinion mechanism, axial displacement of a rack is restricted by a rack stopper (end stopper). There is also an upper limit value for the steering angle of the steering wheel in accordance with the upper limit value of the steered angle. Normally, the steered angle and the steering angle are positive when they are rotation angles in a predetermined direction, and are negative when they are rotation angles in the opposite direction. In the following description, the expression “the steering angle and the steered angle are large” means that a variation from the neutral position is large. In other words, the expression “the steering angle and the steered angle are large” means that the absolute values of the steering angle and the steered angle that can have positive and negative values are large.
A process in which a reaction force is applied to the steering wheel immediately before the absolute value of the steered angle reaches its upper limit value in order to control an increase in steered angle is well known in the art. This process is what is called an end abutting process (steering limiting process). For example, Japanese Pat. No. 4,725,132 describes a steering limiting process in a steer-by-wire system that steers the steered wheels by steering actuators in accordance with the operation of the steering wheel while cutting off power transmission between the steering wheel and the steered wheels.
The steering wheel is equipped with an airbag, a horn, etc. As well known in the art, electrical wiring for these components is connected via a spiral cable, a signal cable formed in a spiral shape, to electronic equipment fixed to a vehicle body. Japanese Patent Application Publication No. 2006-176102 (JP 2006-176102 A) describes that the length of a spiral cable is set so that a steering can be operated until a rack shaft contacts a rack stopper when the steering angle ratio, namely the ratio of the steered angle to the steering angle, is minimum (see FIG. 13).
The length of the spiral cable is not necessarily set to the length described in JP 2006-176102 A. The following problems occur in the case where the length of an actual spiral cable is shorter than that described in JP 2006-176102 A. In the following description, the steering angle ratio is defined as the steered angle divided by the steering angle.
When the steering limiting process is performed in response to the steering angle approaching its upper limit value that is determined by the length of the spiral cable, the situation is avoided where the steering is operated so as to increase the steering angle to a value larger than its upper limit value. In this case, however, if the steering angle ratio is high, the steering limiting process may not be performed even when the rack shaft contacts the rack stopper, because the steering angle has not reached its upper limit.
On the other hand, in the case where the steering limiting process is performed based on the steered angle, the steering limiting process can be performed before the rack shaft contacts the rack stopper, even if the steering angle ratio is high. In this case, however, if the steering angle ratio is low, the steering angle may reach its upper limit value that is determined by the length of the spiral cable, even if the steered angle has not reached its upper limit. In this case, the steering limiting process may not be performed even if the steering angle reaches its upper limit value.
Not only in the steering devices including a spiral cable and a rack and pinion mechanism, but also in the steering devices in which the steered angle and the steering angle have separate upper limit values, the operation of the steering which causes the steered angle or the steering angle to approach its upper limit value and to exceed its upper limit value may not be able to be limited.